Patients receiving Prochymal had a significant and durable improvement
in cardiac function.

"This study adds convincing long-term data to the excellent safety
profile of Prochymal, having now treated hundreds of patients in trials
over the past decade," said C. Randal Mills, Ph.D., President and CEO of
Osiris Therapeutics. "We are excited that Prochymal demonstrated strong
evidence of efficacy beyond the best cardiac care available today. We
are now advancing this program into a larger Phase II trial, focusing on
patients with more severe heart damage."

Recently, Osiris completed enrollment in a Phase III trial of Prochymal
for the treatment of steroid-refractory acute graft versus host disease
(GvHD). In January, clearance from U.S. Food and Drug Administration
(FDA) was received to broaden the expanded access program for Prochymal,
now making the investigational stem cell product available to adults, as
well as pediatric patients, with life-threatening GvHD. In 2008, Osiris
and Genzyme Corp. announced a strategic alliance for the development and
commercialization of Prochymal. Under the terms of the agreement, Osiris
will commercialize Prochymal in the United States and Canada, and
Genzyme will commercialize the treatment in all other countries.

Final Safety Data

Throughout the two-year trial Prochymal demonstrated a favorable safety
profile. In this fully-immunocompetent patient population, there were no
signs of adverse immune response or infusional toxicities. The adverse
events rate experienced by Prochymal patients was lower than that of
placebo patients (8 versus 11 per patient). Notably in the class of
chest pain events, there were fewer cases in the Prochymal group than
placebo (24.2% vs. 63.2%, p= 0.008). No serious adverse events were
attributed to Prochymal, and all-cause hospitalizations trended lower in
the Prochymal group (38.2%) as compared to the placebo group (47.4%).

Performance Data

Data from this trial strongly suggests that Prochymal is able to
attenuate adverse effects typically associated with acute myocardial
infarction by blocking scar formation and pathological remodeling. This
was evident in cardiac performance measures of both electrical and
mechanical function of the heart.

During the trial patients receiving Prochymal experienced fewer
arrhythmias. This effect was maintained for the duration of the study,
with 47.4% of placebo patients experiencing cardiac arrhythmia compared
to only 11.8% of Prochymal patients (p=0.006). Ventricular arrhythmias
are associated tissue damage and scar formation in the heart resulting
from infarction and can be a sign of poorer prognosis.

Along with the conduction effects seen, the mechanical performance data
collected from magnetic resonance imaging (MRI) data on a subset of
patients showed functional recovery that was maintained through the
two-year study.

Left ventricular ejection fraction (LVEF), which reflects the fraction
of blood pumped out of a ventricle with each heart beat, is a common
measurement of overall heart function and typically declines after a
heart attack. Patients with significantly compromised LVEF progress to
heart failure. Two-year MRI data demonstrates there was statistically
significant improvement in LVEF over baseline, 6.6 point in Prochymal
relative to a 3.9 point improvement in placebo. For patients with more
severe myocardial infarction, defined as a baseline LVEF of 45% or less,
even greater effects were observed. The Prochymal group showed a
significant 9.5 point improvement over baseline two years post-treatment
(p < 0.05). This compares favorably to the 3.1 point increase observed
for the placebo group (NS).

"This placebo controlled study was truly first of its kind and the data
produced is promising," said Timothy Henry, M.D., Director of Research
at the Minneapolis Heart Institute Foundation at Abbott Northwestern.
"It clearly suggests that allogeneic adult stem cells have significant
potential to improve recovery following a heart attack and can prevent
long term adverse effects. Given the fact that we can administer this
drug through a standard IV in an acute setting, Prochymal could become
an integral part of standard of care for treatment of heart attacks
everywhere."

About the Phase II Acute Myocardial Infarction Trial

The Phase II double-blind, placebo-controlled trial will evaluate the
safety and efficacy of Prochymal in conjunction with standard of care
for improving heart function in patients who experienced a first heart
attack. This trial focuses on patients who have suffered a severe
myocardial infarction, defined as LVEF between 30% and 45% at baseline.
The target enrollment is 220 patients. Patients will be randomized to
either Prochymal or placebo at 1:1. Efficacy endpoints determined from
cardiac MRI include end systolic volume, LVEF and the ability of
Prochymal to preserve functional heart tissue, or limit scar formation
following a heart attack. In addition, functional and quality of life
assessments will be performed.

About Prochymal

Prochymal is a preparation of mesenchymal stem cells specially
formulated for intravenous infusion. The stem cells are obtained from
the bone marrow of healthy adult donors. Prochymal is currently being
evaluated in Phase III trials for steroid refractory GvHD, acute GvHD,
and Crohn's disease. Prochymal has been granted Fast Track status by FDA
for all three of these indications. Prochymal also obtained Orphan Drug
status by FDA and the European Medicines Agency for GvHD. Prochymal is
being studied in Phase II trials for the treatment of COPD, type 1
diabetes, and acute myocardial infarction. Additionally, the Department
of Defense recently awarded Osiris a contract to develop Prochymal as a
treatment for acute radiation syndrome.

About Osiris Therapeutics

Osiris Therapeutics, Inc. is the leading stem cell therapeutic company
focused on developing products to treat serious medical conditions in
the inflammatory, orthopedic and cardiovascular areas. The Company's
pipeline of internally developed biologic drug candidates under
evaluation includes Prochymal for inflammatory, autoimmune, and
cardiovascular indications, as well as Chondrogen for arthritis in the
knee. Osiris is a fully integrated company, with capabilities in
research, development, manufacturing, and distribution of stem cell
products. Osiris has a partnership with Genzyme Corp. for the
development and commercialization of Prochymal and Chondrogen in
countries outside the United States and Canada. Osiris has developed an
extensive intellectual property portfolio to protect the company's
technology including 47 U.S. patents each having one or more foreign
counterparts. Osiris, Prochymal and Chondrogen are registered trademarks
of Osiris Therapeutics, Inc. More information can be found on the
company's website, www.Osiris.com.
(OSIR-G)

Forward-Looking Statements

This press release contains forward-looking statements. Forward-looking
statements include statements about our expectations, beliefs, plans,
objectives, intentions, assumptions and other statements that are not
historical facts. Words or phrases such as "anticipate," "believe,"
"continue," "ongoing," "estimate," "expect," "intend," "may," "plan,"
"potential," "predict," "project" or similar words or phrases, or the
negatives of those words or phrases, may identify forward-looking
statements, but the absence of these words does not necessarily mean
that a statement is not forward-looking. Examples of forward-looking
statements include, but are not limited to, statements regarding the
following: our product development efforts; our clinical trials and
anticipated regulatory requirements and the ability to successfully
navigate these requirements; the success of our product candidates in
development; status of the regulatory process for our biologic drug
candidates; implementation of our corporate strategy; our financial
performance; our product research and development activities and
projected expenditures, including our anticipated timeline and clinical
strategy for Prochymal, Chondrogen and our other MSC and biologic drug
candidates; our cash needs; patents and proprietary rights; the safety
and ability of our potential products to treat disease and the results
of our scientific research; our plans for sales and marketing; our plans
regarding our facilities; types of regulatory frameworks we expect will
be applicable to our potential products; and results of our scientific
research. Forward-looking statements are subject to known and unknown
risks and uncertainties and are based on potentially inaccurate
assumptions that could cause actual results to differ materially from
those expected or implied by the forward-looking statements. Risks and
uncertainties related to the Collaboration Agreement with Genzyme
include, among others: typical business transactional risks; risks
related to product development and clinical trial design, performance
and completion; uncertainty of the success of Prochymal and Chondrogen
in clinical trials and their ability to treat disease; Genzyme's early
termination and opt-out rights; the ability of Osiris and Genzyme to
successfully navigate regulatory requirements and to manufacture and
commercialize products; and the uncertainty as to the ability of the
parties to successfully perform under the collaborative arrangement and
for Osiris to earn milestone and royalty payments thereunder. Our actual
results could differ materially from those anticipated in
forward-looking statements for many reasons, including the factors
described in the section entitled "Risk Factors" in our Annual Report on
Form 10-K and Quarterly Reports filed on Form 10-Q, with the United
States Securities and Exchange Commission. Accordingly, you should not
unduly rely on these forward-looking statements. We undertake no
obligation to publicly revise any forward-looking statement to reflect
circumstances or events after the date of this press release or to
reflect the occurrence of unanticipated events.